The overall objectives of this proposal are to improve our basic understanding of how dystroglycan binds laminin, and to explore the effectiveness of expressing LARGE and applying drugs of several classes in treating dystroglycan-related muscular dystrophies. In particular, the role of a novel post-translational phosphate modification - an O-linked mannose that is part of the laminin-binding glycan structure of alpha-dystroglycan - in laminin binding will be investigated. Also, new antibodies that recognize only immature alpha-DG with a terminal phosphate modification will be developed and used to evaluate the post-translational modification status of alpha-DG in tissues or cells collected from individuals with various types of dystroglycanopathies (by immunofluorescence, immunoblotting, and radioisotope-labeling). These studies will improve our understanding of the modifications required for dystroglycan to serve as a laminin receptor, and advance patient diagnosis by improving biomarker correlations with clinical severity. In vitro experiments with patient cells have suggested that overexpression of the glycosyltransferase LARGE can bypass alpha-dystroglycan glycosylation defects in a broad range of dystroglycanopathies. This hypothesis will be corroborated In vivo by assessing the ability to prevent disease in different dystroglycanopathy mouse models through either systemic or muscle-specific expression of the Large transgene. This will provide a better understanding of the feasibility and likelihood of success of broadly applying a LARGE-based therapy to the genetically diverse group of dystroglycanopathies. Furthermore, existing clinical drugs will be tested for their potential to ameliorate disease in dystroglycanopathy mouse models. The group of drugs to be tested includes a premature stop-codon readthrough drug (PTC124), as well as steroids (Prednisone) and PDES inhibitors (Sildenafil); the latter two compounds have already shown promise in the treatment of non-dystroglycan related muscular dystrophies. The effectiveness of these treatments will be explored using a combination of cell culture and in vivo mouse studies and measuring dystroglycan function, muscle physiology and pathology. The proposed research will provide a platform for new therapeutic avenues, some of which it will be possible to implement directly in the care of dystroglycanopathy patients